1. Field of the Invention
The present invention relates to an ultra directional speaker system and a signal processing method thereof, and in particular to an ultra directional speaker system and a signal processing method thereof wherein a novel signal processing scheme is employed to improve a sound quality of the speaker system.
2. Description of Prior Art
Generally, a speaker generates a sound by converting an electrical signal to a vibration to be transmitted to an air. The speaker transmits the vibration to the air isotropically. Accordingly, an audience may hear the sound generated by the speaker from all directions with respect to the speaker. The isotrope of the speaker often causes an unnecessary problem. For instance, when various art works or exhibits are displayed in an art gallery or a museum such that a description thereof is provided by the speaker, an interference occurs between sounds generated by the speaker due to a small space of the art gallery and the museum. Moreover, when a number of people listen to the description of different art works or exhibits simultaneously, a large amount of voices are interfered and distorted to be converted to a large amount of noise. In order to solve above-described problem, an ultra directional speaker wherein the sound is reproduced such that the sound is audible in a certain direction has been proposed.
A conventional ultra directional speaker employs a parabolic dish. In accordance with the parabolic ultra directional speaker, a general speaker is disposed at a focus of the parabolic dish such that an acoustic output of the speaker is reflected and travels straight. Since the parabolic ultra directional speaker is frequently used in the museum, the parabolic ultra directional speaker is known as a museum speaker. However, in accordance with the conventional ultra directional speaker using the parabolic dish, a sound quality thereof is poor and a diameter of the parabolic dish is relatively large. And also a distance for a travel of the sound with a direction is only 10 m in the conventional ultra directional speaker.
Therefore, an ultrasonic speaker technology using a non-linear interference of an ultrasonic wave in the air is applied to an embodiment of the ultra directional speaker. While the ultrasonic speaker technology has been developed from 1960s, a commercialization thereof has been delayed until recent years due to a slow development of peripherals and an industrial margin.
The ultra directional speaker comprises a signal processor for obtaining a proper sound quality, a modulator for efficiently modulating a processed signal to an ultrasonic band, an ultrasonic amplifier for driving an ultrasonic converter, and a ultrasonic converter for actually generating an ultrasonic wave in the air. Theoretically, an audible signal p(t) demodulated in the air is proportional to a second-order differentiated square of an envelop signal E(t) of an amplitude-modulated signal as expressed in equation 1, where a is a constant. A second order time partial differentiation in the equation 1 may be solved using 12 dB/octave equalizer, and the according envelop signal E(t) may be expressed as equation 2:p(t)=a∂2/∂t2{E(t)2}  [Equation 1]E(t)=1+mx(t)  [Equation 2]
where m is a modulation index and x(t) is an original audible audio signal.
In accordance with the equations, when the audible signal p(t) audible through the speaker is proportional to the original audible audio signal x(t), a reproduction of the audible sound without any distortion is possible. However, the distortion corresponding to the square of original audible audio signal x(t) as expressed in the equation 1 is seriously generated. While the modulation index m is decreased in the conventional ultrasonic speaker to reduce the distortion, a reproduction efficiency is degraded so that a high acoustic output cannot be obtained.
Another method for compensating the distortion is to modulate a square root of the original signal as shown in FIG. 1. Theoretically, in accordance with the method, the original signal is perfectly reproduced. However, a spectrum of the original signal x(t) which has a limited bandwidth due to a non-linear operation of the square root appears in an almost infinite bandwidth. Therefore, unless an ultrasonic converter that reproduces the infinite bandwidth exists, the ultrasonic speaker shown in FIG. 1 has an absolute limitation in reducing the distortion.
In order to solve the problem of the speaker shown in FIG. 1, American Technology Corporation proposed a repetitive error compensation method without increasing a bandwidth titled “Modulator Processing for a Parametric Speaker System” (U.S. Pat. No. 6,584,205) as shown in FIG. 2. In brief, the patent owned by American Technology Corporation discloses a method wherein an ideal modulated signal waveform is calculated through a SSB (“Single Side Band”) channel model without a converter and an error is calculated by comparing the ideal signal and the actually modulated signal to compensate the error for a signal prior to the modulation, thereby compensating for the distortion of the sound quality. However, since the patent of American Technology Corporation repeatedly compensates for the error, it is disadvantageous in that a large amount of calculation is required for the repeated error compensation such that a hardware design is complex and a delay according to a signal processing is increased. Moreover, since the patent of American Technology Corporation employs the SSB modulation, a sharp SSB filter should be designed by increasing an order thereof in order to prevent the distortion due to an imperfection of the SSB filter.